Method for the preparation of carbonated hydroxyapatite compositions

ABSTRACT

A process for the preparation of a single phase carbonate-substituted hydroxyapatite composition, which process comprises the steps of (i) preparing an aqueous solution containing Co 3   2−  and PO 4   3−  ions in the substantial absence of cations other than H +  ions: (ii) mixing the solution from step (i) with an aqueous solution or suspension of a calcium compound; and (iii) collecting and drying the precipitate formed in step (ii); the ratio of Ca/P in the calcium-containing solution or suspension and the phosphorus-containing solution, when mixed together, being maintained above 1.67. The product of the process is novel with a Ca/P molar ratio of greater than 1.67 and comprises up to 5% by weight of CO 3   2−  ions substituted in the B site or the B and A sites of the hydroxyapatite structure, with at least 50% of the CO 3   2−  ions being substituted on the B site. This product does not contain Na +  or NH 4   +  ions.

The present invention relates to a method for forming a carbonatedhydroxyapatite composition and, in particular, to a method for formingcarbonated hydroxyapatite compositions which are substantiallysodium-free and ammonium-free.

Synthetic hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ has been reported as havingbeen used as a bone replacement material in porous, granular, plasmasprayed and dense forms. Investigations have shown hydroxyapatite to besimilar structurally to bone material. However hydroxyapatite is one ofthe range of stoichiometric calcium phosphate apatites. Human and animalbone mineral have been shown to contain significant amounts of from 3 to7 wt% of carbonate. There is evidence that the carbonate group cansubstitute in two sites, the phosphate and hydroxyl sites, termed B andA respectively; bone mineral being predominantly a B type apatite. As aresult of this similarity in chemical composition, it is envisaged thatcarbonated hydroxyapatite will have better bioactivity thanunsubstituted stoichiometric hydroxyapatite which is currently used incommercial applications, such as plasma-sprayed coatings on metallicimplants and porous hydroxyapatite ceramic bone substitutes. A carbonatesubstituted hydroxyapatite would also find application for use inchromatography and for purification, such as the removal of heavy metalions by adsorption.

The preparation of carbonate-substituted hydroxyapatite ceramicmaterials must be easy and reproducible in order to achieve commercialexploitation. Additionally, the carbonate-substituted hydroxyapatitecomposition must be thermally stable such that it will not decompose toundesirable secondary phases (e.g. tricalcium phosphate or calciumoxide) upon calcining/sintering. Furthermore, during this heattreatment, the carbonate-substituted hydroxyapatite must not lose thecarbonate ions that have been substituted into the hydroxyapatitestructure.

Up to the present time, the methods which have been reported to preparecarbonate-substituted hydroxyapatite compositions have involved one ofthe following procedures.

The heating of a stoichiometric hydroxyapatite ceramic composition in aCO₂ atmosphere at approximately 900° C. for several days [R. Wallaeys,Silicon, Sulphur, Phosphates. Coll. Int. Union Pure Appl. Chem. Münster(1954) 183-190]. This process results in low levels of carbonatesubstitution, with poor control over the extent of carbonatesubstitution and the homogeneity of the substitution throughout thesample. Furthermore, the carbonate substitution is at the wrong site,i.e. the A site, to provide a material which is equivalent to bone.

A wet precipitation method using Na₂CO₃, NaHCO₃ or (NH₄)₂CO₃ as a sourceof carbonate ions results in the substitution of the additional ions,Na⁺ or NH₄ ⁺, into the hydroxyapatite structure, poor thermal stabilityof the product upon calcining/sintering, the loss of large quantities ofthe carbonate ions upon heating, and poor control of the levels of thecarbonate substitution. See, for example: Y. Doi, Y. Moriwaki, T. Aoba,M. Okazati, J. Takahashi and K. Joshin, “Carbonate apatites from aqueousand non-aqueous media studied by esr, IR and X-Ray Diffraction: Effectof NH₄ ⁺ ions on crystallographic parameters”, J. Deut. Res. 61(1982)429-434. D. G. A. Nelson and J. D. B. Featherstone, “Preparationanalysis and characterization of Carbonated apatites”, Calcif. Tiss.Int. 34(1082) 569-581.

EP-A-0722773 and JP-A-8225312 disclose the preparation of an A-typesubstituted hydroxyapatite in which the carbonate ions substitute forOH⁻ ions in the structure.

EP-A-0626590 discloses the preparation of a carbonate substitutedapatite in which the Ca/P ratio is maintained at approximately 1.66 andsodium and carbonate ions are co-substituted into the lattice with theamount of carbonate that is substituted being controlled by the amountof sodium bicarbonate used in the reaction.

WO-A-94/08458 discloses a process for the preparation of carbonatedhydroxyapatite in which the starting materials are mixed at roomtemperature and the material sets to form a cement at room orphysiological temperature. The source of carbonate ions is solid calciumcarbonate. The material produced is poorly-crystalline or amorphousapatite which contains sodium ions.

JP-A-61151011 discloses adding Ca(OH)₂ and CaCO₃ to a slurry of CaHPO₄.The CO₃ ions are introduced into the reaction mixture as insoluble CaCO₃and not via solution. The ratios of Ca/P used are always less than 1.67.After sintering at 1000° to 1100° C. the carbonate content of theresulting material is less than 0.1%.

It is mainly due to the problems encountered with the preparation routesdiscussed above that these routes have not been developed to preparecarbonate-substituted hydroxyapatite ceramic materials commercially.

We have now developed a novel process for the preparation of a singlephase carbonate-substituted hydroxyapatite composition which overcomesthe problems of the prior art methods and does not contain sodium orammonium ions.

Accordingly, the present invention provides a process for thepreparation of a carbonate-substituted hydroxyapatite, which processcomprises the steps of

(i) preparing an aqueous solution containing CO₃ ²⁻ and PO₄ ³⁻ ions inthe substantial absence of cations other than H⁺ ions:

(ii) mixing the solution from step (i) with an aqueous solution orsuspension of a calcium compound; and

(iii) collecting and drying the precipitate formed in step (ii);

the ratio of Ca/P in the calcium-containing solution or suspension andthe phosphorus-containing solution, when mixed together, beingmaintained above 1.67.

The single-phase carbonate-substituted hydroxyapatite compositionsprepared in accordance with the present invention are believed to benovel and, accordingly, in another aspect the present invention providesa single phase carbonate-substituted hydroxyapatite composition, with aCa/P ratio of greater than 1.67, which comprises up to 5% by weight ofCO₃ ²⁻ ions substituted in the B (PO₄) site or the B and A sites of thehydroxyapatite structure, with at least 50% of the CO₃ ²⁻ ionssubstituted on the B site, and which does not contain Na⁺ or NH₄ ⁺ ions.

In carrying out the process of the present invention the aqueoussolution of step (i) may be prepared by bubbling carbon dioxide throughwater to form carbonic acid, and then adding phosphoric acid, H₃PO₄,thereto, or by adding carbon dioxide gas to water under high pressureand then adding phosphoric acid thereto. The amount of carbon dioxideabsorbed by the solution can be calculated from the pH of the solutionprior to the addition of H₃PO₄. At a pH of about 4.0 the solution willbe fully saturated with carbon dioxide. Generally H₃PO₄ will be added tothe solution of carbonic acid in order to provide the PO₄ ³⁻ ions forreaction.

Alternatively, the aqueous solution of step (i) may be prepared bybubbling carbon dioxide through a solution of H₃PO₄ or adding carbondioxide under pressure to the solution, in order to form CO₃ ²⁻ ions insitu. Furthermore, CO₂ may be introduced as a solid which carbonates thesolution as it vaporises.

The solution from step (i) of the process is mixed in step (ii) with anaqueous solution or suspension of a calcium compound. For example, asolution of calcium nitrate, Ca(NO₃)₂, or a suspension of calciumhydroxide, Ca(OH)₂, may be used. Preferably the mixing will be carriedout by dropwise addition of the solution from step (i) to thecalcium-containing solution or suspension. However, bulk mixing of thesolution and the suspension may be undertaken provided that the combinedmixture is vigorously stirred in order to provide the precipitationreaction.

During the mixing in step (ii) of the process carbon dioxide may bebubbled through the mixture.

The ratio of Ca to P in the calcium-containing solution or suspensionand the phosphorus-containing solution, when mixed together, ismaintained at above 1.67 in order to promote substitution in both the Aand B sites to give an AB-substituted hydroxyapatite if having theformula:

Ca₁₀(PO₄)_(6−x)(CO₃)_(x)(OH)_(2−y)(CO₃)_(y)

Preferably the Ca/P ratio is maintained in the range from above 1.67 to1.84, more preferably from above 1.67 to 1.76.

After the addition of the reactants is complete the pH of the mixturemay be adjusted, if desired, to pH 10 to 11 by the addition of ammonia.If ammonia is added in this manner then appropriate steps are taken toremove the ammonia from the final product.

The dried precipitate from step (iii) of the process may becalcined/sintered in a wet carbon dioxide atmosphere according to theteaching of EP-0625490B. In particular, the dried precipitate may becalcined in carbon dioxide containing from 0.001 to 0.10 of grams ofwater per liter of gas at a temperature in the range of from 700° to1200° C., preferably from 900° to 1200° C. Preferably the carbon dioxideused as the sintering atmosphere will contain from 0.01 to 0.02 grams ofwater per liter of gas. The sintering time will generally be up to 24hours, preferably 10 minutes to 4 hours.

The sintering will generally be carried out at atmospheric pressure,i.e. no imposed pressure, although pressures slightly higher thanatmospheric may be produced by the particular configuration of thefurnace used.

The carbonated hydroxyapatite compositions produced according to theprocess of the present invention will generally comprise up to 5% byweight of CO₃ ²⁻ ions, preferably from 3 to 5% by weight. Furthermore,the carbonated hydroxyapatite composition will generally have from 50 to85% of the CO₃ ²⁻ ions substituted on the B site.

The carbonated hydroxyapatite compositions produced according to theprocess of the present invention are prepared in the substantial absenceof cations other than H⁺ and Ca²⁺. Accordingly, the compositions do notcontain other cations, such as Na⁺ or NH₄ ⁺, substituted in theirstructures, and thus are biocompatible. The carbonated hydroxyapatitecompositions prepared in accordance with the present invention may beused in any of the applications for which hydroxyapatite is used, forexample the formation of plasma-sprayed coatings on metallic implants,the formation of porous ceramic bone substitutes, the preparation ofcomposites with polymeric materials such as high density polyethylene,as granules or beads for packing or filling bone defects, as materialsfor use in chromatography or as materials for use in purificationmethods such as the removal of heavy metals by adsorption.

The present invention is further described hereinbelow with reference tothe accompanying drawings, in which:

FIGS. 1 and 2 show X-ray diffraction data for the compositions ofExamples 1 to 5, and for unsubstituted hydroxyapatite having a Ca/Pratio of 1.67;

FIG. 3 shows the effect of sintering temperature on the sintered densityof unsubstituted hydroxyapatite and the carbonate-substitutedhydroxyapatite of Example 3;

FIG. 4 shows the microhardness of unsubstituted hydroxyapatite ascompared to the carbonate-substituted hydroxyapatite of Example 3 atdifferent temperatures;

FIG. 5 is the FTIR spectrum of unsubstituted hydroxyapatite sintered at900° C. in air; and

FIG. 6 is the FTIR spectrum of the carbonate-substituted hydroxyapatiteof Example 3 sintered at 900° C. in CO₂/H₂O.

The present invention will be further described with reference to thefollowing Examples.

EXAMPLES 1 TO 5

A suspension of calcium hydroxide was prepared by dispersing 38.25 g ofCa(OH)₂(AnalaR, BDH), in 1 liter of deionised water. This suspension wasstirred for 15 minutes prior to further reaction to form solution A.

Carbon dioxide (CO₂) gas was bubbled into 0.75 liters of deionised waterover a period of 30 minutes during which time the pH of the solutiondecreased from approximately 7 to approximately 4. Phosphoric acid,H₃PO₄, (BDH GPR 85% assay) 0.3 moles (34.588 g) was added to the 0.75liters of CO₂-treated water and this solution was then made up to atotal of 1 liter with deionised water to form solution B.

Solution B was added dropwise to solution A, which was stirredconstantly; the addition of solution B took approximately 3 hours andwas performed at room temperature. After the addition of solution B thepH of the.resulting mixture was adjusted to 10.5-11 with the addition ofapproximately 10 ml of ammonia (BDH AnalaR). The mixture was stirred for2 hours and then aged overnight without stirring. The aged mixture wasfiltered, the filtercake was washed with two 100 ml portions ofdeionised water to remove any residual ammonia and the resultingfiltercake was dried at 80° C. overnight. The dried filtercake wascrushed and ground to a fine powder having an average particle size ofbelow 100 μm.

The procedure detailed above was repeated four times using differentquantities of Ca(OH)₂ to prepare the carbonate-substitutedhydroxyapatite, whilst using the-same amount (0.3 moles) of H₃PO₄.

Further details are given in Table 1 below:

TABLE 1 Example No Ca(moles) Ca(OH)₂(g) Ca/P ratios 1 0.516 38.250 1.722 0.540 40.013 1.80 3 0.528 39.124 1.76 4 0.522 38.679 1.74 5 0.53439.568 1.78

The carbonate-substituted hydroxyapatites prepared as described abovewere thermally stable to 900° to 1200° C. in a CO₂/H₂O atmosphere. Abovethese temperatures, partial decomposition to hydroxyapatite and calciumoxide/calcium carbonate was observed.

The precipitated and calcined/sintered powders (900° C. CO₂/H₂O werecharacterized by CHN analysis, XRF, XRD and FTIR analysis. FIGS. 1 and 2show X-ray diffraction data for the prepared and calcined/ sinteredcarbonate-substituted hydroxyapatite samples of Examples 1 to 5, and forunsubstituted hydroxyapatite having a Ca/P ratio of 1.67. The X-raydiffraction data does not show any peaks attributable to calcium oxideor tricalcium phosphate which would be obtained as decompositionproducts if the carbonate-substituted hydroxyapatite resulted from theA-type substitution (calcium-rich) or B-type substitution(calcium-deficient) mechanisms. This confirms that thecarbonate-substituted hydroxyapatite is of the AB-type.

The X-ray diffraction patterns of calcined/sintered samples (FIG. 2)with Ca/P ratios of 1.67 to 1.76 show only peaks corresponding to HA.Samples prepared with Ca/P ratios above 1.76 show small levels of CaCO₃with HA as the main phase (>95%). However, lower calcinationtemperatures (i.e. less than 900° C.) will result in no decomposition toCaCO₃.

Fourier Transform Infra-red Spectroscopy (FTIR) analysis allows theidentification of different functional groups, such as OH, PO₄ and CO₃,by their characteristic vibration frequencies; the energy of mostmolecular vibrations correspond to the infrared region of theelectromagnetic spectrum. FTIR spectroscopy is an ideal method fordetermining the presence or absence of different functional groups inhydroxyapatite. Stoichiometric, unsubstituted hydroxyapatite shouldproduce only vibrational bands corresponding to OH and PO₄ groups in theFTIR spectrum. In addition to detecting the CO₃ groups in acarbonate-substituted hydroxyapatite, FTIR spectroscopy should indicatethe effect of the substituted CO₃ groups on the sites that it will beoccupying in the HA lattice i.e. the PO₄ and/or the OH groups.

FTIR spectra for an unsubstituted hydroxyapatite andcarbonate-substituted hydroxyapatite produced in the present invention(Example 3) are shown in FIGS. 5 and 6, respectively. The unsubstitutedhydroxyapatite has a sharp peak at approx. 3571 cm⁻¹ that corresponds toOH groups and the peaks at 1065, 1042, 1023, 961 and 629, 599 and 570cm⁻¹ correspond to PO₄ groups. For the carbonate-substitutedhydroxyapatite, the peaks corresponding to PO₄ bands, at 1083, 1045,1021, 961 and 636, 602, and 584 cm⁻¹, are slightly displaced from thevalues observed for unsubstituted hydroxyapatite. The significantchanges observed in the spectrum for the carbonate-substitutedhydroxyapatite are that the OH peak appears to be of a lower intensitycompared to the other peaks, suggesting less OH groups in thecarbonate-substituted hydroxyapatite. Also, carbonate, CO₃, groups areidentified by peaks at 1542, 1454, 1410 and at 877 cm⁻¹. In addition,small shoulders were observed on both sides of the peak at 1454 cm⁻¹;these peaks were at approximately 1420 and 1480 cm⁻¹. Results of FTIRanalysis of A and B-type carbonate substituted hydroxyapatites reportedin studies by Le Geros et al (Specialia Experimentia 25, 5-7, 1969),Nelson et al (Calcif. Tiss. Int. 34, 569-581,1982) and Doi et al (J.Dent. Res. 61, 429-434, 1982) indicate that the peaks observed for thecarbonate-substituted hydroxyapatite produced in the present invention(Example 3) correspond to an AB-mixed substitution of CO₃ in thehydroxyapatite lattice.

CHN analysis, of the as-prepared carbonate-substituted hydroxyapatitepowders and the resulting heated/sintered material showed that nonitrogen (and therefore, ammonia, NH₃) was present. The amount ofcarbonate in the carbonate-substituted hydroxyapatite powders and theresulting heated/sintered material varied from 0 to approximately 5 wt%, depending upon the Ca/P ratio of the material; higher Ca/P ratiosresulted in more carbonate substitution.

Sintering Data

The carbonate-substituted hydroxyapatites produced in the presentinvention sinter to higher densities for a specific temperature between750 and 1050° C. than unsubstituted hydroxyapatite. FIG. 3 shows theeffect of sintering temperature on the sintered density of unsubstitutedhydroxyapatite and the carbonate-substituted hydroxyapatite produced inExample 3 (sintering conditions as follows: for carbonate-substitutedhydroxyapatite, atmosphere of CO₂/H₂O as described in method, and forunsubstituted hydroxyapatite, atmosphere of air. For both materials, aheating rate of 2.5° C./min to temperature, dwell time of 2 hours, andcooling-rate of 10° C./min to room temperature). The substitution ofcarbonate ions in to the hydroxyapatite lattice results in a powder thatsinters to form a ceramic with a high density at temperatures that areregarded as low for ceramics in general.

The high densities achieved with the carbonate-substitutedhydroxyapatite produced in the present invention result in superiormechanical properties compared to unsubstituted hydroxyapatite, asreflected by microhardness determination, FIG. 4. To determine themicrohardness, an applied load of 0.5 kg was applied to the polishedsurface of a ceramic sample for 10 seconds. The average diagonal lengthof the indent was measured and the microhardness, Hv, was calculatedfrom the method described in ASTM E384. For a given sinteringtemperature, the microhardness of the carbonate-substitutedhydroxyapatite produced in the present invention was significantlygreater than the value for unsubstituted hydroxyapatite Additionally,the reproducibility of the method described in the present invention hasbeen tested. Three repeat batches (50 g) were made with the samestarting Ca/P ratio (1.76). A large-scale batch (150 g) was also made.The results of CHN (wt % carbonate determination) analysis of these fourpowders are listed in Table 2; samples were measured as-prepared(processed powders prior to heat-treatment) and after sintering at 900°C. in a CO₂/H₂O atmosphere.

Heat wt % Sample ID Method Treatment CO₃ Unsubstituted HA Normal pptnas-ppd 1.7 Ca/P = 1.67 900° C. air — Example 3 (i) Present inventionas-ppd 3.7 900° C. CO₂ 3.2 Example 3 (ii) Present invention as-ppd 4.1900° C. CO₂ 3.3 Example 3 (iii) Present invention as-ppd 4.0 900° C. CO₂3.2 Example 3 (iv) Present invention as-ppd 3.6 (large-scale) 900° C.CO₂ 3.1

Preliminary Investigation of the Bioactivity of Carbonate-substitutedHydroxyapatite

The bioactivity of the carbonate-substituted hydroxyapatite produced inthe present invention compared to unsubstituted hydroxyapatite has beenassessed by testing the ceramic specimens in Simulated Body Fluid (SBF),which contains the type and concentration of the ions that are presentin human plasma. This test demonstrates the time required for a newbone-like apatite layer to precipitate on the surface of the testceramic specimen, the shorter the time, the more bioactive the material.The carbonate-substituted hydroxyapatite (Example 3) produced in thepresent invention resulted in the formation of a new bone-like apatitelayer in less than 7 days, whereas the unsubstituted hydroxyapatiterequired 24-28 days. This test provides an early indication of theimproved biological properties that the carbonate-substitutedhydroxyapatite produced in the present invention may offer compared tounsubstituted hydroxyapatite.

The results from these analyses indicate that the carbonate-substitutionis a combined AB-type substitution, described by the equation:

Ca₁₀(PO₄)_(6+x)(CO₃)_(x)(OH)_(2+y)(CO₃)_(y)

with at least 50% of the CO₃ ²⁺ being substituted on the B site of thehydroxyapatite structure.

What is claimed:
 1. A process for the preparation of a calcined singlephase carbonate-substituted hydroxyapatite composition, which processcomprises the steps of (i) preparing an aqueous solution containing CO₃²⁻ and PO₄ ³⁻ ions in the substantial absence of cations other than H⁺ions: (ii) mixing the solution from step (i) with an aqueous solution orsuspension of a calcium compound; (iii) collecting and drying theprecipitate formed in step (ii); and (iv) calcining the driedprecipitate from step (iii) in carbon dioxide containing from 0.001 to0.10 grams of water per liter of gas at a temperature of from 700° to1200° C. the ratio of Ca/P in the calcium-containing solution orsuspension and the phosphorus-containing solution, when mixed together,being maintained above 1.67.
 2. A process as claimed in claim 1 whereinthe aqueous solution of step (i) is prepared by bubbling CO₂ throughwater to obtain a solution of carbonic acid-and then adding H₃PO₄thereto.
 3. A process as claimed in claim 2 wherein the carbonic acidsolution has a pH of about 4 before and addition of H₃PO₄.
 4. A processas claimed in claim 1 wherein the solution from step (i) is addeddropwise with stirring to the aqueous solution or suspension of thecalcium compound.
 5. A process as claimed in claim 1 wherein a calciumnitrate solution or a calcium hydroxide suspension is used in step (ii).6. A process as claimed in claim 1 wherein CO₂ is passed through thesolution from step (i) during mixing step (ii).
 7. A process as claimedin claim 1 wherein the ratio of Ca/P in the calcium-containing solutionor suspension and the phosphorus-containing solution, when mixedtogether, is in the range of from above 1.67 to 1.76.
 8. A process asclaimed in claim 1 wherein the carbonated hydroxyapatite compositioncomprises up to 5% by weight of CO₃ ²⁻ ions.
 9. A process as claimed inclaim 2 wherein the solution from step (i) is added dropwise withstirring to the aqueous solution or suspension of the calcium compound.10. A process as claimed in claim 3 wherein the solution from step (i)is added dropwise with stirring to the aqueous solution or suspension ofthe calcium compound.
 11. A process as claimed in claim 2 wherein acalcium nitrate solution or a calcium hydroxide suspension is used instep (ii).
 12. A process as claimed in claim 3 wherein a calcium nitratesolution or a calcium hydroxide suspension is used in step (ii).
 13. Aprocess as claimed in claim 4 wherein a calcium nitrate solution or acalcium hydroxide suspension is used in step (ii).
 14. A process asclaimed in claim 2 wherein CO₂ is passed through the solution from step(i) during mixing step (ii).
 15. A process as claimed in claim 3 whereinCO₂ is passed through the solution from step (i) during mixing step(ii).
 16. A calcined carbonate-substituted hydroxyapatite wheneverprepared by a process which comprises the steps of (i) preparing anaqueous solution containing CO₃ ²⁻ and PO₄ ³⁻ ions in the substantialabsence of cations other than H⁺ ions: (ii) mixing the solution fromstep (i) with an aqueous solution or suspension of a calcium compound;(iii) collecting and drying the precipitate formed in step (ii); and(iv) calcining the dried precipitate from step (iii) in carbon dioxidecontaining from 0.001 to 0.10 grams of water per liter of gas at atemperature of from 700° to 1200° C. the ratio of Ca/P in thecalcium-containing solution or suspension and the phosphorus-containingsolution, when mixed together, being maintained above 1.67.
 17. Acalcined single phase carbonate-substituted hydroxyapatite composition,with a Ca/P molar ratio of greater than 1.67, which comprises up to 5%by weight of CO₃ ²⁻ ions substituted in the B (PO₄) site or the B and Asites of the hydroxyapatite structure, with at least 50% of the CO₃ ²⁻ions substituted on the B site, and which does not contain Na⁺ or NH₄ ⁺ions.
 18. A calcined single phase carbonate-substituted hydroxyapatiteas claimed in claim 17 wherein from 50 to 85% of the CO₃ ²− ions aresubstituted on the B site.
 19. A calcined single phasecarbonate-substituted hydroxyapatite which comprises from 3 to 5% byweight of CO₃ ²⁻ ions substituted in the B (PO₄) hydroxyapatitestructure.